This invention, which is a result of a contract with the U.S. Department of Energy, relates to crystals and methods of making the same having desirable luminescence characteristics for possible use as host materials in tunable lasers.
It has long been known that refractory oxide crystals luminesce when optically excited through exposure to particular wavelengths of light, and this phenomenon is used to advantage in various devices such as lasers, photon detectors, image converters, and other quantum electronic solid state devices.
In general, both impurities and vacancies in a given crystal give rise to optical absorption bands, which when optically excited result in luminescence. These crystals have been used as laser host materials whose outputs may be tuned to yield monochromatic coherent light over a certain wavelength region covered by the luminescence spectrum of the crystal. Lasers using impurities have been much more common because impurities (such as chromium in the classic ruby laser) are thermally more stable than lasers using vacancies. However, vacancies have three main advantages over impurities: (1) The oscillator strength of the absorption is generally much higher; therefore, the required concentration of the lasing species is much lower. For many impurity ions, the oscillator strengths are small (approx. 10.sup.-5) and concentrations of approximately 1% are required. This magnitude can normally be expected to yield complications involving alloying. precipitations and severe grain boundary problems. For vacancies, the oscillator strengths in absorption is typically unity and therefore concentrations of only approximately 0.0001% are adequate. (2) The broader luminescence bands in principle offer a broader tuning range. The half widths of absorption bands and luminescence bands of vacancies are larger than those of impurities, typically a 5:1 ratio. (3) The broad absorption bands permit optical pumping by a correspondingly broad wavelength spectrum of an incoherent exciting source.
However, there are two disadvantages of vacancies in refractory oxide crystal systems (such as MgO, CaO, Al.sub.2 O.sub.3, and SrO) for tunable laser hosts, as compared with impurities: (1) the thermal instability of vacancies produced by conventional methods, such as irradiations with electrons or neutrons, and (2) luminescence in thermochemically reduced crystals is long-lived (greater than approximately 1 second).
In the art it has been the practice to produce vacancies in refractory oxide crystals by exposing single crystals of the oxide to either electron or neutron radiation over an extended period of time at about 300.degree. K. This technique is disclosed by B. Henderson in Optics Letters , Vol. 6, No. 9, pp. 437-439 (September 1981). Vacancies produced in this manner are not stable because they are annihilated at temperatures not much higher than room temperature, thus restricting their use. These crystals must be cooled to prevent annihilation of the vacancies by interstitial-vacancy recombination. Furthermore, cooling is necessary to obtain acceptable quantum efficiency. Another shortcoming of the irradiated crystals is that exciting into the absorption band of the F+ center gives rise to trapped hole centers which are a detriment to continuous-wave (cw) tunable laser action. Irradiated oxide crystals are generally not desirable hosts for cw tunable lasers.
In general, anion vacancies in oxides assume two charge states: the one-electron F+ center and the two-electron F center. While the absorption bands of these two charge states in most oxides occur at different wavelengths, in MgO they coincide at 5.0 eV (250 nm). In MgO, CaO and SrO irradiated with energetic particles, the preferred charge state is the F+ center. Excitation of MgO with 5.0 eV light produces the F+ luminescence at 3.1 eV (395 nm) with a FWHM (full width at half maximum) of 0.6 eV. Such broad emission in principle permits a broad tuning range for laser action. When the electron of an F+ center is in an excited state, the strong polarization at the vacancy demands an electron from a nearby 0.sup.2- ion, thereby leaving behind a hole. The hole migrates and is trapped by a magnesium vacancy charge-compensated by an impurity inherently present in MgO crystals, such as Al.sup.3+ and H+, to form the V.sub.Al or V.sub.OH centers, whose linear configurations are Al.sup.+3 -[Mg vacancy]-O- and OH--[Mg vacancy]-O-, respectively. The O- refers to an O.sup.2- ion with a trapped hole. These trapped hole centers give rise to broad absorption bands with a FWHM of 1.1 eV at 300.degree. K., with peaks at about 2.3 eV (540 nm). These centers are metastable and have a half-life for hole-release of a few hours at 300.degree. K. They obviously are a strong deterrent to producing cw lasing action in the region of their optical absorption.
Irradiation with high-energy electrons to produce F+ centers has essentially the same effect and offers no advantage. In fact, the concentration of the trapped-hole centers increases. Substitutional protons in the crystals are displaced by the ionizing electrons with a phenomenally large cross section (10.sup.8 barns), resulting in the intrinsic V- center (linear configuration: O.sup.2- -[Mg vacancy]-O-). This center also absorbs at 2.3 eV but has a half-life for hole release of several months, thereby compounding an already undesirable problem. An absorption coefficient as large as 25 cm.sup.-1 has been observed. Furthermore, electron irradiations are expensive and time consuming. Irradiation levels between 10.sup.18 and 10.sup.19 electrons/cm.sup.2 for many hours in a Van de Graaff generator are required. The sample temperature during irradiation cannot exceed 360.degree. K. without annihilation. This requirement adds to the long irradiation time.
These considerations are not restricted to MgO. Trapped-hole centers in several oxides have been identified by electron paramagnetic resonance studies. They generally give rise to broad absorption bands in the visible and near-visible region. In CaO and Al.sub.2 O.sub.3 the bands peak at 1.85 (670 nm) and 3.0 eV, (400 nm) respectively.
Although neutrons may be used to create vacancies in less time, there still exists the problem of thermal instability. The reason is that interstitials are created during irradiation, and being much more mobile than vacancies they readily recombine with vacancies. However, if there are no interstitials created, as with the thermochemically reduced crystals, the vacancies can survive at much higher temperatures.
In thermochemically reduced crystals there exists a long-lived component of the F center luminescence, which is detrimental for use as a laser host. It is due to the presence of hydride ions (H- ions). Luminescence lifetimes of roughly 10.sup.-8 to 10.sup.-6 seconds are desirable for cw laser hosts. Therefore, in order to provide a refractory oxide host for tunable cw laser applications, hydrogen must be removed from the crystal.
High quantum efficiency, high oscillator strength and thermally stable refractory oxide laser host crystals can be provided by removing the hydrogen from a high purity refractory oxide crystal and subsequently thermochemically reducing the crystal to produce oxygen (anion) vacancies therein. Details of such a method may be had by referring to U.S. Pat. No. 4,604,225, issued Aug. 5, 1986 to Chen et al for "Refractory Oxide Hosts For a High Power, Broadly Tunable Laser With High Quantum Efficiency And Method Of Making Same", and having a common assignee with the present invention. The contents of the above referenced patent being incorporated herein by reference thereto.
As described in the above referenced patent, hydrogen may be removed from a high purity single crystal of the refractory oxide by irradiating the crystal with an ionizing radiation or heating the crystal to break the OH- bonds while applying an electric field to the crystal to force the freed protons to be swept to the cathode electrode. Following the hydrogen removal, the crystals are subjected to a thermochemical reduction process to stoichiometrically reduce the oxygen ions, thereby creating oxygen anion vacancies due to the deficiency of oxygen ions without oxygen interstitials.
While thermochemical reduction is an advantages means of preparing refractory oxide crystals for laser host materials, there are two serious problems: (1) long-lived phosphorescence from anion vacancies, and (2) crystal darkening. In thermochemically reduced oxide crystals containing low hydrogen concentrations, there still exists a component of the F+ luminescence which is low in intensity but can persist for periods as long as about one second. This phosphorescence component, in excess of 1 microsecond, can constitute more than 80% of the integrated luminescence intensity and is therefore detrimental to achieving laser action. In order to make the material useful, it is necessary to transform this long-lived component into short-lived (nanosecond) luminescence.
Further, thermochemically reduced oxide crystals tend to have broad absorption bands introduced by impurity metal precipitates that cause darkening of the crystal even in highly pure crystals. In MgO crystals this darkening appears to be due to the presence of iron precipitates.
Thus, there is a need to provide an improved refractory oxide crystal for use as a host material for tunable lasers and a method of preparing the same.